Mobile telephones are conventionally known as portable communication apparatuses.
Especially, for an IMT-2000 telephone among the mobile telephones, an IC card called a SIM (Subscriber Identification Module) card is distributed to each user.
Each SIM card stores subscriber identification information such as the telephone (subscriber) number and carrier (the carrier for which the user has signed up, and its communication method) of each sign-up for a line. When the SIM card is attached to the telephone, and the identification information is read out from the SIM card, the user can perform communication based on an outgoing call or incoming call. The user can also selectively use a plurality of telephones by exchanging the SIM card to be attached to a telephone.
When one user has signed up for a plurality of communication lines, SIM cards each storing identification information for one communication line are provided. Hence, the user can selectively use the plurality of communication lines in one telephone by exchanging the SIM card to be attached to the telephone.
A telephone to which two SIM cards are attachable has also been proposed (e.g., Japanese Patent Laid-Open No. 2003-189351). When a user has signed up for two lines and received two SIM cards, he/she can perform communications using the two lines in one telephone.
A mobile telephone of this type executes cell search at the time of power-on to search for a communicable radio base station and register the position. When the mobile telephone moves out of a zone where it can communicate with a radio base station, the telephone periodically executes cell search and, upon returning into the zone, quickly shifts to the standby state.
As described above, if the cell search execution interval of the mobile telephone is set long outside the zone, the shift to the standby state in returning into the zone is delayed. On the other hand, if the cell search interval is short, the base station search process is frequently executed, resulting in an increase in battery consumption of the mobile telephone.
Especially, in a mobile telephone attached with a plurality of SIM cards and capable of communicating using a plurality of carriers, as described in Japanese Patent Laid-Open No. 2003-189351, it is necessary to execute cell search and position registration as many as the carriers to be used.
This further increases power consumption as compared to a conventional mobile telephone that uses only one carrier.